The sigma-delta circuit is a “building block” used in innumerable systems. Basically a sigma-delta circuit is a feedback circuit that generates an output digital replica of the difference between two analog input signals and is used in numerous functional circuits designed for specific applications. Most often sigma-delta circuits are used in signal converters from the analog domain to the digital domain and vice versa, in modulators, digital filters and other circuits.
In PWM applications and the like (commonly known as PWM or PWM-like systems), the sigma-delta stage or modulator is upstream of or is associated to a PWM or PWM-like quantizer stage, that in PWM applications substitutes the single bit or multiple bit quantizer that is used for implementing a converter (for example an ADC), and the feedback loop of the sigma-delta architecture is closed by a delay block 1/z, suitable for introducing a delay equal to a fractional value of the period of the PWM signal, as depicted in FIG. 1.
FIG. 1 is an exemplary embodiment of a first-order sigma-delta PWM modulator with single bit output, the input signal of which is processed together with a signal proportional to the signal output by the output modulator, delayed by a time established by the delay block (1/z). Such a PWM modulator often drives a power stage, for example a loudspeaker of an acoustic system, a motion actuator and the like.
FIG. 2 illustrates a sigma-delta structure of a third-order sigma-delta PWM modulator with single bit output. To minimize distortion and because of electrical characteristics of the power transistors of output bridge stages, there may be constraints on the minimum pulse duration of PWM signals delivered to the load, as depicted in the diagram of FIG. 3. This may imply constraints on amplitudes of signals that may be correctly converted to PWM signals. In these conditions, when the amplitude of the signal input to the system decreases, the signal that inputs the PWM or PWM-like modulation quantizer may be so small to be correctly transferable in form of a PWM signal to the load of the system.
An approach disclosed in literature includes prolonging the duration of the pulse to be transferred by a time equal to the minimum transferable pulse and subtracting therefrom, immediately after, a quantity equal to the minimum duration of such a transferable pulse. In this way, an otherwise untransferable analog signal may be correctly transferred in PWM form. This approach is illustrated in FIG. 4. A drawback of this approach is a decrement of the efficiency of the output power stage because it is forced to switch more times than would be ideally necessary.